linux-kernel - Re: [PATCH 02/25] mm: Introduce mm_fault

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Date:   Tue, 16 Jun 2020 12:00:37 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 02/25] mm: Introduce mm_fault_accounting()

On Mon, 15 Jun 2020 19:19:17 -0400 Peter Xu <peterx@...hat.com> wrote:

> On Mon, Jun 15, 2020 at 03:32:40PM -0700, Linus Torvalds wrote:
> > On Mon, Jun 15, 2020 at 3:16 PM Peter Xu <peterx@...hat.com> wrote:
> > >
> > > Provide this helper for doing memory page fault accounting across archs.  It
> > > can be defined unconditionally because perf_sw_event() is always defined, and
> > > perf_sw_event() will be a no-op if !CONFIG_PERF_EVENTS.
> > 
> > Well, the downside is that now it forces a separate I$ miss and all
> > those extra arguments because it's a out-of-line function and the
> > compiler won't see that they all go away.
> > 
> > Yeah, maybe some day maybe we'll have LTO and these kinds of things
> > will not matter. And maybe they already don't. But it seems kind of
> > sad to basically force non-optimal code generation from this series.
> 
> I tried to make it static inline firstly in linux/mm.h, however it'll need to
> have linux/mm.h include linux/perf_event.h which seems to have created a loop
> dependency of headers.  I verified current code will at least generate inlined
> functions too for x86 (no mm_fault_accounting() in "objdump -t vmlinux") with
> gcc10.
> 
> Another alternative is to make it a macro, it's just that I feel the function
> definition is a bit cleaner.  Any further suggestions welcomed too.

Could create a new header file mm_fault.h which includes mm.h and
perf_event.h.  A later cleanup could move other fault-related things
into that header and add the appropriate inclusions into files which
use these things.

btw, I think mm_account_fault() might be a better name for this function.

And some (kerneldoc) documentation would be nice.  Although this
function is pretty self-evident.

> > 
> > Why would you export the symbol, btw? Page fault handling is never a module.
> 
> I followed handle_mm_fault() which is exported too, since potentially
> mm_fault_accounting() should always be called in the same context of
> handle_mm_fault().  Or do you prefer me to drop it?

Let's not add an unneeded export.  If someone for some reason needs it
later, it can be added then.